Ductile Cast Iron

Ductile Cast Iron

Cast iron is an alloy of iron, carbon and silicon with carbon levels of about 3.2% and silicon at about 2.2%. If molten cast iron is allowed to cool normally the carbon forms flakes of graphite which run through the iron matrix, hence the term flake graphite iron. These flakes are at the microscopic level, the ends of which form stress points in the cast iron. If cast iron is subject to a compressive load these stress points are not particularly detrimental and flake graphite cast iron is excellent under compressive load. However, tensile loading above the natural tensile strength of the cast iron can cause rapid tensile failure as cracks propagate rapidly out from these stress points.

The result of this is that cast iron has virtually no elongation, is a brittle material and is therefore limited in its use in tensile and shock loading applications. For years foundry men and metallurgists tried to develop a new type of cast iron that would withstand bending and shock loading and would have the characteristics more of malleable cast iron but could be produced at the lower cost of grey cast iron. In 1943, in the International Nickel Company Research Laboratory, Keith Dwight Millis made a ladle addition of magnesium (as a copper-magnesium alloy) to cast iron – the solidified castings contained not flakes, but nearly perfect spheres of graphite. Ductile iron was born. The advantage of ductile iron is that the spheres of graphite don’t act as stress raisers but as crack arresters and are what give ductile iron its ductility. This new form of cast iron immediately found uses where malleable iron, forgings, cast steel or steel fabrications would have been used. From this start, ductile cast iron has grown into a world class material offering cast solutions at a competitive price compared to traditional alternatives.

Ductile iron is a family of cast graphitic irons which possess high strength, ductility and resistance to shock. Annealed cast ductile iron can be bent, twisted or deformed without fracturing. Its strength, toughness and ductility duplicate many grades of steel and far exceed those of standard gray irons. Yet it possesses the advantages of design flexibility and low cost casting procedures similar to gray iron. The difference between ductile iron and gray iron is in the graphite formation. Ordinary gray iron is characterized by a random flake graphite pattern in the metal. In ductile iron the addition of a few hundredths of 1% of magnesium or cerium causes the graphite to form in small spheroids rather than flakes. These create fewer discontinuities in the structure of the metal and produce a stronger, more ductile iron. It is this graphite formation which accounts for the fact that ductile iron is also referred to as “nodular iron.” produces valves, pumps, and other process equipment in ASTM A395 ductile iron.

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